1. Field of the Invention
This invention relates to a low voltage creeping discharge igniter plug having a semi-conductor ring to form a creeping discharge surface, and particularly concerns to an igniter plug in use for a gas or turbine engine of aircraft.
2. Description of the Prior Art
In an igniter plug of this kind, a semi-conductor ring is placed between both end portions of a center electrode and an annular ground electrode, so that a discharge occurs through an end surface of the semi-conductor ring at a relatively low voltage.
For this discharge constantly to occur, it is necessary to diminish a resistance between the electrodes at their lower end so as to concentrate a discharge energy thereupon.
By way of illustration, as shown in FIG. 7, a semi-conductor ring 13 which is connected in integral with a tubular insulator 14 at their lower portion, is provided between a center electrode 12 and a ground electrode 11b. The ground electrode 11b provides an inner tapered surface 11a at the lower portion in a manner that a distance between an outer surface of the center electrode 12 and the tapered surface 11a progressively increases as being successively away from the ends of the electrodes 11b, 12. The least distance is obtained at the ends of the electrodes 11b, 12, so that a discharge occurs along an end surface 13a of the semi-conductor ring 13. It is noted that a very minute gap is provided between the center electrode 12 and the ground electrode 11b as a clearance although not shown.
In this construction, an atmosphere near the end surface 13a increases an electrical resistance, failing to occur the discharge along the end surface 13a when used at a high atmospheric condition. A voltage impressed between the electrodes 11b, 12 causes to discharge within the semi-conductor ring 13, fearing to crack the semi-conductor ring 13 so as to result in a breakdown of insulation.
Therefore, it is a primary object of this invention to provide a low voltage creeping discharge igniter plug which is capable of maintaining a discharge to occur along an end surface of a semi-conductor ring without breakdown of the semi-conductor, leading to a long service life.
According to this invention, there is provided a low voltage creeping discharge igniter plug comprising; an outer annular metallic shell having an annular ground electrode connected to a lower end thereof; a tubular insulator concentrically placed into the metallic shell and having the semi-conductor ring connected to a lower end of the insulator; a center electrode longitudinally positioned within an inner side of the insulator and a lower end of the center electrode extending through the semi-conductor ring; the semi-conductor ring forming a creeping discharge surface at the lower end when a certain magnitude of voltage is applied between the ground electrode and the center electrode; an annular space provided between an inner surface of the semi-conductor ring and an outer surface of the center electrode, a distance therebetween progressively increasing as being away from the creeping discharge surface.
Other objects and advantages of this invention will be apparent from the following description, reference being made to the accompanying drawings where-in preferred embodiments of this invention are shown.